"If you have 1% of your population infected, and you have a test that's only 99% specific, that means that when you find a positive, 50% of the time will be a real positive and 50% of the time it won't be," Birx said.
In other words, if the prevalence of infection in the community is 1%, about half of all positive tests will be false positives. Depending on the manufacturer, the test may return even higher rates of false results.